The present invention relates generally to semiconductor devices for sensing chemical concentration or activity, and more particularly to a method for fabricating ion-sensitive field-effect transistor devices having a nonplanar structure and the devices produced thereby.
In recent years there has been a growing need for compact sensors of chemical concentration and activity, particularly for use in aqueous solutions. As a result of such need there has been considerable research and development effort in the area of chemically sensitive semiconductor devices. Much of this effort has centered around a device called the ion-sensitive field-effect transistor (herein ISFET). The operation of the ISFET will now be briefly explained with the aid of FIG. 1, which shows a known ISFET structure fabricated with a conventional planar silicon technology similar to that used for producing metal-oxide-semiconductor field-effect transistors (MOSFET).
Referring now to FIG. 1, the ISFET device 10 produced by the conventional technology includes a relatively lightly doped P-type (P.sub.-) monocrystalline silicon body region 11 into which are diffused two relatively heavily doped N-type (N.sub.+) regions 12 and 13. The body region 11 may be a portion of a substrate wafer or an epitaxially grown layer. The regions 12 and 13, which respectively serve as the source and drain regions of the device, are diffused into the body 11 from its top surface 14. A portion of that surface 15 between the source and drain regions 12 and 13 is covered with a relatively thin, thermally grown silicon dioxide layer 16. The remainder of the top surface of the device 10 is covered with a relatively thick silicon dioxide layer 17, through which contact apertures 8 and 9 are opened to expose the surfaces of the source and drain regions 12 and 13, respectively. Strip-like metal layers 6 and 7 are then deposited over the source and drain regions 12 and 13, respectively, making ohmic contact to those regions through the contact apertures 8 and 9. The metal layers 6 and 7 serve as the source and drain contacts, respectively, while the silicon dioxide layer 16 serves as the ion-sensitive insulating layer of the ISFET device 10.
In the absence of an electric field at the silicon surface 15 beneath the oxide layer 16, the ISFET device 10 exhibits an extremely high electrical resistance between its source and drain contacts 6 and 7. If the device 10 is immersed in an aqueous solution, the silicon dioxide layer manifests hydrating properties similar to that of the glass membrane of the well known glass electrode PH measuring device. If the solution contains cation activity, a double charge layer forms at the interface of the solution and the oxide layer 16, establishing an electric field at the silicon surface 15. In the event that the electric field at the silicon surface 15 becomes sufficiently large, an inversion layer or channel is induced in the silicon body 11 adjacent to the surface 15 causing the resistance between the source and drain contacts 6 and 7 of the device 10 to decrease. The threshold electric field at which the channel is induced depends on the surface doping concentration of the silicon body 11, which may be adjusted during fabrication of the device 10. Since the conductance of the channel depends on the electric field at the silicon surface 15, which in turn depends on the ion activity in the solution, the resistance between the source and drain contacts 6 and 7 provides a measure of the ion activity in the solution.
The ISFET provides two major advantages over other known ion activity sensors, such as ion selective electrodes. Firstly, a reference electrode is not required when measurements are made with the ISFET. Secondly, the responsive variable of an ISFET device is a relatively low, ohmic resistance, which can be easily measured with precision.
However, the ISFET device fabricated by conventional methods, such as that illustrated in FIG. 1, has a serious shortcoming in that its planar structure presents formidable packaging problems. Owing to the source and drain contacts 6 and 7 of the device being on the same side of the substrate chip as the ion-sensitive insulating layer 16, it is very difficult to reliably isolate the contacts 6 and 7 from the solution into which the layer 16 is immersed. Isolation of the source and drain contacts 6 and 7 from the chemically active solution is of critical importance not only for avoiding an undesirable parasitic conducting path through the solution, but also for preventing corrosion of the contacts by the solution, which can lead to failure of the device. Accordingly, a need clearly exists for a method of fabricating ISFET devices which provides device structures amenable to effective and reliable isolation of the source and drain contacts of the device from the solution into which the ion sensitive portion of the device is immersed.
Holes have been drilled in semiconductor wafers heretofore.
Three articles of the present inventor deal with the subject of wafers and laser drilled holes in such wafers. They are as follows:
(1) "Forming electrical interconnections through semiconductor wafers" by T. R. Anthony, J. Appl. Physics, 52 (8), 5340 (1981).
(2) "Forming Feedthroughs in Laser-Drilled Holes in Semiconductor Wafers by Double-Sided Spattering" by T. R. Anthony, IEEE Trans. CHMT-5 (1), 1971 (1982).
(3) "Diodes formed by Laser Drilling and Diffusion" by T. R. Anthony, J. Appl. Phys. 53 (12), 9154 (1982).